Thursday, August 11, 2011

The family Herpesviridae comprises over 120 viruses that infect wide range of vertebrates, including fish, amphibians, birds, and other mammals including humans. Herpesviruses have large enveloped virions, double strand DNA.

The members of this family include herpes simplex virus types 1 and 2, Varicella zoster virus (VZV), Cytomegalovirus, Epstein barr virus, and Human Herpes Viruses (HHVs) 6, 7 and 8. The typical clinical course of all the infections consists of a primary infection, which may be symptomatic, followed by repeated reactivations, often asymptomatic. Nosocomial infections are rare and usually the result of direct inoculation from person to person. VZV is the only member of this family affecting humans that has the potential to be transmitted by aerosols. Immunocompromised patients e.g. transplant recipients are susceptible to reactivations and, because of the immunosuppression, are more likely to have symptomatic disease

A. Herpes simplex viruses (HSV):

The most common clinical manifestation of HSV infections include oral (Predominantly HSV-1), genital (Predominantly HSV-2), ulceration, keratitis, neurological disease, encephalitis, meningitis, herpetic whitlow and neonatal infection. The immunocompromised can also develop life-threatening disseminated infections. Direct contact either with lesions or saliva is the most efficient way of transmitting HSV. Contamination of the hands is common in individuals with herpes labials. This route has been implicated in cross-infection outbreaks. Transmission from patient to staff is also well recognized. Before gloves were worn routinely in dental practice, herpetic whitlows often resulted from direct inoculation of the virus into the fingers following contact with infected saliva in the patient's mouth.

Schematic drawing of Herpes virus

B. Varicella zoster virus (VZV):

Varicella zoster virus is the cause of chickenpox and shingles. Primary infection (chickenpox) is usually a mild self-limiting, the majority of cases are children. Complications are rare with <2 deaths per 100,000 cases. In adult the disease is more severe, with a higher incidence of pneumonitis, encephalitis, and death. Severe disease is also more common in the immunocompromised; mortality rates may approach 15%. Protection is life-long, and reinfection is rare.

Varicella zoster virus spread by contact with the infected lesions, articles contaminated with vesicle fluid, droplet spread from respiratory secretions (chicken pox). VZV is highly infectious, with 80% attack rate in the household setting. Nosocomial outbreaks have been well described with aerosolization of the virus. The importance of this route was confirmed by Sawyer et al., (1994) who detect VZV DNA in air samples up to 2.5 meters from the beds of patients with chickenpox and even outside the patients' room.

Patients with chickenpox are infectious via the respiratory route from 2 days before the rash appears until it crusts over (usually about 5 days). Symptoms usually develop 14 days following an exposure (range, 10-21 days). Patients with uncomplicated chickenpox or shingles should be nursed at home, in severe cases chickenpox should be placed in isolation. Isolation of shingles case is also preferable, but if no single rooms are available and the affected dermatome doses not involve an exposed part of the body, this may not be necessary.

Standard respiratory precautions are recommended. Following identification of a case of VZV infection in the hospital, the infection control team should be informed, to coordinate follow-up of staff and patients. Those patients and staff who have had close contact with index case should have their VZV immune status determined. For patients and staff found to be seronegative, prophylaxis with varicella zoster immune globulin (VZIG) is available especially for immunocompromised patients including BMT patients for up to 6 months post transplantation. VZIG is ideally should be given within 96 hours of exposure. It may, however, still to be of benefit up to 10 days after exposure.

If possible all seronegative patients should be discharged home or isolated between days 10 and 21 following exposure and seronegative members of staff should be either redeployed or sent home during the incubation period. In U.S., varicella vaccination is now part of the routine immunization schedule for infants 12 to 18 months. Advisory committees on immunization practice (ACIP) recommend its use for susceptible adults who have close contact with high-risk patients. Postvaccination testing of hospital staff is not required, as 99% of adults seroconvert after 2 doses given 4-8 weeks apart.

Although there is evidence to suggest that immunization within 72h of varicella exposure may offer protection in children, there is no data to support its use in adults, and there has been no formal trial of the currently licensed vaccine for this use. Therefore, varicella vaccine is not currently recommended for prophylaxis of adult contacts exposed to nosocomial varicella.